The Fischer-Tropsch reaction refers to the reaction of converting a syngas (H2+CO) into hydrocarbons and other chemicals in the presence of a catalyst at certain temperature under certain pressure. In recent years, with the decreasing petroleum resources and constantly increasing price of crude oils, Fischer-Tropsch synthesis has aroused a worldwide interest among energy researchers. Generally speaking, the Fischer-Tropsch synthesis process can be represented by the following formulae:mCO+(2m+1)H2→CmH2m+2+mH2O  (1)mCO+2mH2→CmH2m+mH2O  (2)2mCO+(m+1)H2→CmH2m+2+mCO2  (3)
The Fischer-Tropsch reaction is typically carried out in a Fischer-Tropsch reactor, which can be a fixed-bed reactor or a slurry bubble column reactor (SBCR). Since the fixed-bed reactor is more expensive than the slurry bubble column reactor, and Fischer-Tropsch reaction is an exothermic reaction whose reaction temperature can hardly be controlled, the slurry bubble column reactor is advantageous over the fixed-bed reactor.
There is no doubt that the slurry bubble column reactor has many advantages over the other reactors, and draws increasing attention from the public. As a gas-liquid-solid multiphase reactor, the slurry bubble column reactor (SBCR) has the advantages of simple structure, large liquid holdup, small temperature gradient, large heat capacity, satisfactory heat transfer performance, easy to control the temperature, easy to treat solid particles and low operating cost.
However, the catalyst in the slurry bubble column reactor is seriously abraded, so that there are a quantity of the fine powders or dusts of the catalyst in the liquid product and it is rather difficult to separate the fine powders from the liquid product, which undoubtedly increases the complexity and operating cost of the industrial Fischer-Tropsch synthesis reactor. Moreover, the broken catalyst particles usually cause the forced shutdown of the subsequent filtration apparatus, such that the running period of the whole system may be sharply shortened.
In fact, in the slurry bubble column reactor, the turbulent fluid movement of gas-liquid-solid three-phase reaction streams are rather strong, including the motions of large bubbles, small bubbles, and the slurry and catalyst particles. Different stream exhibit different hydrodynamic characteristics, such as advection, eddy current and turbulence, in different regions. Meanwhile, cooling tubes and other structures inside the reactor can cause the solid catalyst particles to break up at a rather rapid speed through collision and friction in such a fluid environment. In most cases, after a period of time, the particle size thereof will be reduced from the dozens of microns to in the beginning to sub-microns fine powders.
The magnetic force formed by the magnetic field is increasingly applied to mixing, separating, filtrating, or even steady flowing of the multiphase streams. For example, U.S. Pat. No. 3,219,318 discloses a fluid-stirring apparatus, wherein there are many non-spherical permanent magnet parts distributed in the fluid, and the periphery of the fluid is provided with a magnetic field with alternating intensity and alternative direction to enable the aforesaid non-spherical permanent magnet parts to be involved in rotation and parallel displacement in the fluid, so as to achieve the object of stirring the fluid.
US2010/0113622 discloses a system for separating liquids from solids comprising an immobilization unit comprising an immobilization vessel containing a bed of magnetizable material and a magnet configured to produce a magnetic field within the immobilization vessel, wherein the immobilization vessel further comprises an immobilization vessel outlet and an immobilization vessel inlet for a fluid comprising liquid and metal-containing particles. When the fluid comprising a liquid and the magnetic solid particles flows through the aforesaid immobilization vessel, most of the solid particles in the fluid are removed in the bed of magnetizable material. The aforesaid system may be used to separate liquid from solid catalyst particles and may be particularly applied in multi-phase catalytic reactors where the catalyst comprises magnetic solids particles, for example, in the removal or filtration of the residual catalyst particles comprised in the liquid product in the Fischer-Tropsch (FT) reactors.
U.S. Pat. No. 4,296,080 discloses a fluidized bed process for removing impurity particles from the fluid, wherein the fluidized bed is a moving controllable particle trap bed, comprising magnetic particles. The fluidized bed is extended and lifted by the fluid streams, and can be controllably moved according to the pressure differential of the fluidized bed, wherein at least part of the region in the fluidized bed is applied with a magnetic field wherein the major magnetic component is in the so-called direction of the external force field. The magnetic field intensity has to be sufficient for suppressing the solid backmixing and fluid shunt appearing in the fluidized bed, but less than the value weakening the fluid characteristics of the fluidized bed. When the fluid containing magnetic impurity particles passes through the aforesaid particle trap bed, most of the impurity particles are trapped and removed.
US Re. 31186 discloses a fluidized bed process for hydrocarbon conversion, wherein the fluidized bed comprises the magnetic and fluidizable composite particles having the catalytic activity towards the conversion of hydrocarbons, as well as 2-40 vol % of iron or iron magnetic materials. A constant and substantially uniform magnetic field is applied to the aforesaid fluidized bed along the direction of gravity to allow the composite particles to have a magnetic force along the direction of gravity, and allow the fluidized gaseous medium comprising the gasifying hydrocarbon feed to pass through the fluidized bed upwardly at an apparent velocity which is greater than at least 10% of the conventional minimum value of the apparent gas velocity required by the fluidized bed when the magnetic field is not applied, but less than the apparent gas velocity required by the fluctuations of the pressure differential as time changed when passing through said fluidized bed within the interval of 0.1 to 1 second with the magnetic field applied. The aforesaid fluidized bed enables the fluidized state of each stream to be present in a stable and homogenous condition, and especially to eliminate the generation of large bubbles in the fluidized bed.
To sum up, when a slurry bubble column reactor is used in the Fischer-Tropsch reaction, how to avoid the serious attrition of catalyst particles and to highly effectively separate the catalyst fine powders or dusts from the liquid product is a serious process issue troubling the industry for a long time without a satisfactory solution so far.
Based on intensive research, on the fluid hydrodynamics inside the slurry bubble column reactor and the mechanism of the electromagnetic field, the inventor successively developed a magnetically assisted slurry bubble column reactor suitable for the Fischer-Tropsch reaction and capable of sharply reducing the attrition of catalyst particles in the Fischer-Tropsch reaction. The magnetically assisted slurry bubble column reactor in the present invention can be used to resolve the problem of liquid-solid filtration in a three-phase Fischer-Tropsch reactor, and to enhance the mass transfer between the catalyst particles and reactant gases as well as the service life of the catalyst.